Field of the Invention
The present invention relates to a nickel-cobalt-manganese composite hydroxide which serves as a precursor for a positive electrode active material of a non-aqueous electrolyte secondary battery, and a production method therefor, and more particularly, relates to a nickel-cobalt-manganese composite hydroxide which serves as a precursor for a positive electrode active material of a lithium ion secondary battery, and a production method therefor. The present application claims priority based on the Japanese Patent Application No. 2013-128887 filed on Jun. 19, 2013 in Japan, which is incorporated by reference herein.
Description of Related Art
Conventionally, smaller and lighter secondary batteries which have high energy densities have been required with popularization of mobile devices such as cellular phones and laptop personal computers. Batteries preferred for these applications include lithium ion secondary batteries, which have been actively researched and developed.
Furthermore, also in a field of automobiles, a demand for electric vehicles has been increased from resource and environmental perspectives, and lithium ion secondary batteries which are small and light, high in discharging capacity, with favorable cycle characteristics have been required as power sources for electric vehicles and hybrid automobiles. In particular, for power sources for automobiles, output characteristics are important, and lithium ion secondary batteries have been required which have favorable output characteristics.
Lithium ion secondary batteries that use, for positive electrode active materials, lithium-containing composite oxides, in particular, relatively easily synthesized lithium-cobalt composite oxides (LiCoO2) achieve high voltages on the order of 4 V grade, and have been thus progressively put into practical use as batteries with high energy densities. Further, a large number of lithium ion secondary batteries that use this type of lithium-cobalt composite oxides have ever been developed for achieving excellent initial capacity characteristics and cycle characteristics, and various results have been already achieved.
However, the lithium-cobalt composite oxides cause, because of the use of expensive cobalt compounds for raw materials, increases in the cost of active materials and thus batteries, and improved active materials have been desired. The battery that uses the lithium-cobalt composite oxide has a significantly higher unit price per capacity than a nickel-hydrogen battery, and thus has a considerably limited use application. Therefore, there are great expectations to lower the cost of active materials, thereby allowing the manufacture of more inexpensive lithium ion secondary batteries not only small-size secondary batteries for currently popular portable devices, but also for large-size secondary batteries, e.g., for electricity storage, and for electric vehicles, and the achievement of the manufacture can be considered to have industrially enormous significance.
In this regard, 4 V-grade positive electrode active materials that are more inexpensive than lithium-cobalt composite oxides, that is, lithium-nickel-cobalt-manganese composite oxides that have a composition of Li[Ni1/3Co1/3Mn1/3]O2 with nickel:cobalt:manganese substantially of 1:1:1 in atomic ratio have been attracting attention as new materials of positive electrode active materials for lithium ion secondary batteries. The lithium-nickel-cobalt-manganese composite oxides have been actively developed, because of not only their inexpensiveness, but also because of exhibiting higher thermal stability than lithium ion secondary batteries that use a lithium-cobalt composite oxide or a lithium-nickel composite oxide for a positive electrode active material.
In order for lithium ion secondary batteries to provide favorable battery characteristics, the lithium-nickel-cobalt-manganese composite oxides as positive electrode active materials need to have moderate particle sizes and specific surface areas, and also have high densities. These properties of the positive electrode active materials strongly reflect the properties of nickel-cobalt-manganese composite hydroxides as precursors, and similar properties are thus required for the nickel-cobalt-manganese composite hydroxides.
Furthermore, in order to obtain positive electrode active materials that provide favorable battery characteristics, nickel-cobalt-manganese composite hydroxides are required which are likely to cause reactions with lithium compounds to proceed even in an atmosphere of a water vapor or a carbon dioxide gas generated during syntheses with the lithium compounds, and excellent in reactivity. Nickel-cobalt-manganese composite hydroxides which are poor in reactivity with lithium compounds make reactions with nickel-cobalt-manganese composite hydroxides incomplete during syntheses with the lithium compounds, thereby producing residual unreacted lithium compounds. In addition, there is the problem of melting the lithium compounds, thereby causing agglomeration, before completing the reactions between the nickel-cobalt-manganese composite hydroxides and the lithium compounds.
In regard to nickel-cobalt-manganese composite hydroxides to serve as precursors for positive electrode active materials, various suggestions have been made as will be described below. However, while the increase in density has been examined in each of the suggestions, surface properties or the reactivity with lithium compounds have not been taken into full account.
For example, Patent Literature 1 proposes continuously supplying an aqueous solution of a nickel salt, which contains a cobalt salt and a manganese salt, a complexing agent, and an alkali metal hydroxide, in an inert gas atmosphere or in the presence of a reductant in a reaction vessel, and continuously extracting crystals obtained through continuous crystal growth, thereby providing a spherical high-density cobalt-manganese co-precipitated nickel hydroxide that has a tap density of 1.5 g/cm3 or more, an average particle size of 5 to 20 μm, and a specific surface area of 8 to 30 m2/g.
The co-precipitated nickel hydroxide obtained can be used as a raw material for lithium-nickel-cobalt-manganese composite oxides. However, this co-precipitated nickel hydroxide has, according to an example, a tap density of 1.71 to 1.91 g/cm3, which is less than 2.0 g/cm3, which can be thus considered to be an insufficiently high density. On the other hand, any specific numerical value is not mentioned for the specific surface area, it is not clear whether the specific surface area is made appropriate or not, and the reactivity with lithium compounds has not been examined. Therefore, even when this co-precipitated nickel hydroxide is used as a precursor, any lithium-nickel-cobalt-manganese composite oxide will not be obtained which has favorable battery characteristics.
In addition, Patent Literature 2 suggests a method for producing a lithium-nickel-cobalt-manganese composite oxide, which includes: a step 1 of reacting and co-precipitating mixed aqueous solution of a nickel salt, a cobalt salt, and a manganese salt with nickel:cobalt:manganese of substantially 1:1:1 in atomic ratio in the presence of a complexing agent in an aqueous solution, with pH of 9 to 13 with an alkali solution under an inert gas atmosphere, thereby providing a nickel-cobalt-manganese composite hydroxide and/or a nickel-cobalt-manganese composite oxide with nickel:cobalt:manganese of substantially 1:1:1 in atomic ratio; and a step 2 of firing a mixture of the hydroxide and/or the oxide with a lithium compound at 700° C. or higher so that the atomic ratio of lithium to the total atomic ratio of nickel, cobalt, and manganese is substantially 1:1. Also in this proposal, the obtained nickel-cobalt-manganese composite hydroxide has a tap density of 1.95 g/cm3, which is less than 2.0 g/cm3, and a very large specific surface area of 13.5 m2/g. Moreover, the reactivity with lithium compounds has not been examined either.
Accordingly, nickel-cobalt-manganese composite hydroxides have been desired which have favorable reactivity with lithium compounds, and make it possible to produce nickel-cobalt-manganese composite oxides such that favorable battery characteristics are achieved.